We study the transcriptional control of T cell development and function. T cells are essential for immune responses. Most of them recognize peptide antigens presented by class I (MHC-I) or class II (MHC-II) classical Major Histocompatibility Complex molecules, and express either of two surface glycoproteins (called coreceptors) that contribute to antigen recognition: CD4, which binds MHC-II, or CD8, which binds MHC-I. Consistent with such binding properties, MHC I-specific T cells generally are CD4-CD8+ (CD8 T cells), whereas MHC II-specific T cells generally are CD4+CD8- (CD4 T cells). CD4 and CD8 T cells differentiate in the thymus from precursors that express both CD4 and CD8 ('double-positive', DP). In addition to their distinct antigenic specificity, CD4 and CD8 T cells perform different functions upon antigen encounter: whereas CD8 T cells typically differentiate into cytotoxic effectors, CD4 T cells provide help to other components of the immune system (and of mucosal barriers). Additionally, a subset of 'regulatory' CD4 T cells expressing the transcription factor Foxp3 (Treg cells) contributes to suppress immune responses. Because of their pivotal role in immune responses (they are the key target of the human immunodeficiency virus HIV), the differentiation of CD4 T cells has remained a key area of focus in the laboratory. We had previously shown that the transcription factor Thpok promotes CD4 T cell development in the thymus, and the stability of mature, post-thymic, CD4 T cells, the latter in part redundantly with the related transcription factor LRF (Leukemia-Lymphoma Related Factor). Furthermore, we had also shown that Thpok and LRF are essential for the emergence of helper effector functions in MHC II-restricted thymocytes and for the maintenance of such functions in post-thymic CD4 T cells, including for proper immune responses in vivo. Ongoing analyses focus on the mechanisms underlying the role of Thpok in immune responses. CD4 T cells are also essential for the maintenance of immune tolerance. Indeed, in addition to a profound immune-depression, innate or acquired CD4 T cell deficiency in humans is often accompanied by manifestations of auto-immunity, which reflect the critical role of Foxp3-expressing CD4 Treg cells for immune homeostasis. Contrasting with these observations, the CD4 T cell deficiency caused by Thpok disruption is not associated with auto-immunity, suggesting that Thpok, while needed for the development and function of 'conventional' Foxp3-negative CD4 T cells, is dispensable in Foxp3-positive Treg cells. However, it was also possible that persistent immune tolerance despite Thpok disruption reflected balanced contributions of this factor to the development and functions of Foxp3-negative and -positive CD4 T cells. We have used two approaches to distinguish between these possibilities. First, we have examined the differentiation of Treg cells from Thpok-deficient thymocytes. Second, to study the impact of Thpok on the functions of differentiated Treg cells, we have inactivated the gene encoding Thpok specifically in Treg cells. Because of the partial overlap between the functions of Thpok and LRF, we also inactivated LRF, or both Thpok and LRF, in thymocytes and mature Treg cells. We found that Thpok, with little functional overlap with LRF, is needed for the differentiation of thymic Treg precursors, an observation in line with the fact that Foxp3-expressing Tregs are invariably CD4 cells. Gene inactivation in mature Treg cells found that the Thpok-LRF transcriptional 'node' is needed for Treg cell function. Indeed, Treg-specific disruption of Thpok and Lrf causes a lethal inflammatory syndrome similar to that resulting from Treg cell deficiency. In contrast, inactivation of either factor had little if any impact on Treg function and immune tolerance, indicating greater functional overlap than in the thymus. Additional analyses showed that Thpok and LRF support interleukin 2- andFoxp3-mediated gene expression, and have therefore a broad impact on the Treg transcriptome. Functionally, they promote the survival of Treg cells and are necessary for the generation of effector Treg cells. Last, unlike in conventional T cells, Thpok and LRF functions in Treg cells are not mediated by their repression of the transcription factor Runx3. In summary, these studies show that Thpok and LRF are needed for regulatory MHC II-restricted T cell responses. Together with our previous reports that Thpok and LRF support the differentiation and function of conventional Foxp3-negative CD4 T cells, these new findings demonstrate that the transcriptional 'node' formed by Thpok and LRF controls both the activating (helper) and suppressive arms of MHC II-restricted immune responses. We are currently investigating the mechanisms of Thpok and LRF functions in developing and mature CD4 T cells, including the transcriptional circuitry driving CD4-lineage differentiation in the thymus, by interrogating the impact of these factors on gene expression and chromatin organization.